四明十義書 1.71

1.71

且如《止觀》十境，攝一切所觀，何曾將佛界心不為境？恐是大師明境有不了之過，故上人特釋之也。

Furthermore, the ten objects of the Zhiguan encompass all that is contemplated; when did it ever not treat the mind of the Buddha realm as an object? I fear your reverence thinks the Great Master was guilty of not being thorough in clarifying the objects, and that is why you offered this special explanation!